1. Field of the Invention
The present invention relates generally to multilayer circuit boards, and more particularly to multilayer circuit boards having air bridge crossover circuits.
2. Disclosure Information
U.S. Pat. No. 4,404,059 to Livshits et al. (hereinafter xe2x80x9cLivshitsxe2x80x9d), U.S. Pat. No. 3,801,388 to Akiyama et al. (hereinafter xe2x80x9cAkiyamaxe2x80x9d), and U.S. Pat. No. 5,738,797 to Belke et al. (hereinafter xe2x80x9cBelkexe2x80x9d), all three of which are incorporated herein by reference, describe additive and subtractive plating and chemical etching processes for constructing multilayer (i.e., two-or-more-layer) circuit boards having air bridges. As typified in FIGS. 1A and 2A-F (and in FIGS. 1a-h of Livshits), the additive process involves the following steps. First (FIG. 2A), a thin metal foil 10 made of a first metal is provided having opposed top and bottom surfaces 12/14. Second (FIG. 2B), top and bottom etch-resist masks 22/24 are disposed on the respective top and bottom foil surfaces, wherein the top and bottom masks have apertures 23/25 therein which define respective first and second conductor patterns. Third (FIG. 2C), top and bottom conductor networks 32/34 made of a second metal are electrodeposited through the mask apertures and onto the respective top and bottom foil surfaces. Fourth (FIG. 2D), the masks are stripped off, fifth (FIG. 2E), the structure is attached to an electrically insulative surface 61 of a substrate 60, and sixth (FIG. 2F) the structure is exposed to an etchant which etches substantially only the first metal, so as to etch away substantially all of the metal foil except for those portions 16 sandwiched between the top and bottom conductor networks. These sandwiched portions 16 are much less aggressively etched because the top and bottom conductor networks act essentially as etchresist masks, since the metal of these networks is not appreciably etched by the aforementioned etchant. After etching, these sandwiched portions 16 typically assume an hour-glass-like shape, and may be referred to as xe2x80x9cpedestalsxe2x80x9d,  xe2x80x9csupport elementsxe2x80x9d, or the like.
Air bridge crossover circuits are advantageous in that they facilitate high density circuitsxe2x80x94that is, they permit the arrangement of a high number of circuit traces and other circuit elements into a small circuit board area. However, such circuits sometimes suffer from at least one of the following two problems: (1) susceptibility to mechanical damage, from impact, thermal excursions, and the like; and (2) poor electrical performance, caused by the use of certain metals having poor electrical conductance characteristics as the conductor network material. It would be desirable, therefore, to provide a way of creating such multilayer air bridge circuits without these drawbacks.
The present invention overcomes the disadvantages of the prior art approaches by providing a multilayer air bridge circuit structure, and method for producing the same, which has special metallic fortifying layers which mitigate the aforementioned disadvantages.
It is an object and advantage that the present invention includes such fortifying layers made of a metal selected such that the air bridge structure is mechanically and/or electrically fortified.
Another advantage is that the present invention may be easily and economically incorporated into the conventional air bridge circuit construction process.
These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow.